A subject matter of the invention is a process for potentiating and stimulating the natural defenses of plants of the type of those which are agronomically useful.
It relates in particular
to the vine and fruit trees of the group comprising the apple tree and the pear tree,
to cereals of the group comprising wheat, maize and rice,
to oleaginous plants of the group comprising soya, sunflower and rape, and
to vegetable plants of the group comprising carrots, cauliflower, tomatoes and potatoes.
The stimulation of the natural defenses of plants is one of the most topical problems and forms the subject of a great deal of research.
The stimulation of the natural defenses is reflected, in the case of a plant which has had an initial contact with a pathogenic agent, such as a virus, a bacterium, a fungus or an insect, by the development of a set of biological modifications which confer, on this plant, a presensitization by virtue of which it becomes capable of reacting more effectively to a fresh attack.
The research studies discussed above have made it possible to demonstrate that the presensitization can be obtained by bringing the plant into contact with certain synthetic chemicals.
Furthermore, products called elicitors are known which, when brought into contact with the plant, are capable of stimulating, in the latter, defensive reactions of the type:
accumulation of natural antibiotics better known under the name of phytoalexins,
synthesis of defensive proteins, such as chitinases or glucanases, also known under the name of PRP (Pathogenesis-related proteins),
hardening of the cell walls by synthesis of lignin or of crosslinking proteins,
synthesis of secondary messengers, such as ethylene, hydrogen peroxide or salicylic acid.
Mention may in particular be made among these products, of oligo xcex2 1-3 glucans, which elicit the defensive reactions in question in various agronomically useful plants; the maximum responses are generally achieved when the oligo xcex2 1-3 glucans are employed either in the form of liquid compositions, which comprise them at concentrations of the order of 200 mg/l, and they are maintained at a comparable level up to concentrations of the order of 4 g/l, or in amounts per hectare of 4 to 200 g.
There are a number of disadvantages to proceeding in this way, which results in the deployment of defensive reactions in the absence of any pathogenic agent, among which disadvantages may be mentioned a not insignificant energy expense for the plant.
The aim of the invention is therefore, in particular, to make available to users novel means which make it possible to arrange for the stimulation of the natural defenses of the plant only to take place in the event of need, in other words during attack on the plant by a pathogenic agent, these means consequently conferring the abovementioned acquired systemic resistance, in other words an immunity against pathogenic agents, on the plant.
And it is to the credit of the Applicant Company to have found, as a result of intensive research, that, entirely surprisingly and unexpectedly, not only is this result achieved but that a potentiation of the natural defenses even takes place as soon as a composition is applied to a plant of the type in question comprising one or more oligo xcex2 1-3 glucans composed of 3 to 250, preferably of 3 to 50 and more preferably of 3 to 30 monosaccharide units, their molecular masses consequently ranging from 540 to 45,000 Daltons, these oligo xcex2 1-3 glucans being present in the composition at concentrations lower than those which directly induce defensive reactions, these concentrations being of the order of 1 to 20 mg/l, preferably of 2 to 10 mg/l in the case of wheat and tobacco, which corresponds to the use per hectare of crop of an amount of 1 to 200 g, preferably of 4 to 80 g, of the abovedefined oligo xcex2 1-3 glucans.
This is because it has been able to show that, when the aforesaid oligo xcex2 1-3 glucans are used in the form of compositions in which they are present at concentrations lying within a narrow range from 1 to 20 mg/l in the case of wheat and tobacco, which corresponds to the use per hectare of crop of the amounts indicated above, they exert, on the treated plants, a potentiating effect with regard to the natural defensive reactions, which are only triggered from the time when attack by a pathogenic agent effectively takes place.
The advantage thereof is that the metabolism of the plant is not diverted toward the deployment of defensive reactions at a time when this deployment is pointless; this metabolism is alerted and it is only when the plant is the subject of an attack that it mobilises its defenses, this being done in a much more intense way than in the absence of treatment.
It follows that the process for stimulating the natural defenses of agronomically useful plants is characterized in that it comprises the application, in particular the foliar application, to the plants of the type in question, in particular those from the group defined above, of an effective amount of at least one oligo xcex2 1-3 glucan, this effective amount being lower than those which directly induce defensive reactions.
The oligo xcex2 1-3 glucans employed in the context of the aforesaid process are composed of 3 to 250, preferably of 3 to 50 and more preferably still of 3 to 30 monosaccharide units, their molecular mass consequently ranging from 540 to 45,000 Daltons.
The xe2x80x9ceffective amountxe2x80x9d is defined by the amount of at least one oligo xcex2 1-3 glucan which it is advisable to apply per hectare of crop to be treated in order to confer, on the plants of this crop, the acquired systemic resistance which has just been discussed.
It is, in the case of wheat, from 1 to 200 g, preferably from 4 to 80 g.
In other words, a subject matter of the invention is the use, in potentiating and stimulating the natural defenses of agronomically useful plants, of a plant protection composition comprising one or more oligo xcex2 1-3 glucans composed of 3 to 250, preferably of 3 to 50 and more preferably of 3 to 30 monosaccharide units, the characteristics of concentration of oligo xcex2 1-3 glucans and of use of this composition being chosen so that it contributes, per hectare of crop, an amount of oligo xcex2 1-3 glucans lower than that which directly induces the natural defensive reactions, this amount being in practice from 1 to 200 g, preferably from 4 to 80 g per hectare in the case of cereals and in particular of wheat.
The composition is employed by application, in particular by foliar spraying, on one or two occasions at the early growth stages, in particular at the 2-leaf stage.
The precise moment of the application or applications will be chosen in particular as a function of the plant treated and of its growth stage.
The composition employed is generally aqueous in the case of foliar application; it comprises not only the active substance composed of the oligo xcex2 1-3 glucan or glucans but also the conventional constituents of this type of composition, the concentration of active substance in the composition being from 0.001 to 0.02 g/l, preferably from 0.002 to 0.01 g/l in the case of cereals.
In drawing up the aforesaid composition, it is possible to use, instead of water, a vehicle chosen from the group consisting of mineral and vegetable oils, liquid fatty substances and alcohols, in particular propylene glycol and glycerol.
The conventional constituents of the aforesaid compositions are chosen, according to the plants to be treated, from the group consisting of solvents, surface-active agents, dispersing agents and solid fillers.
The volume of composition employed per hectare is from 10 to 1000 liters and generally of the order of 500 liters.
In an advantageous embodiment, the aforesaid plant protection composition comprises, in combination with the oligo xcex2 1-3 glucans, at least one other plant protection product chosen from the group consisting of fungicides and insecticides.
Another subject matter of the invention is a concentrate in the form of a liquid or in the form of a powder or of granules, suitable for providing, by dilution with an appropriate amount of solvent, in particular of water, the composition used in accordance with the invention.
To prepare the aforesaid oligo xcex2 1-3 glucans, it is possible to hydrolyse xcex2 1-3 glucans, which are polysaccharides with a variable molecular weight of 60,000 to more than 1 million Daltons; these polysaccharides are composed essentially of D-glucose units connected specifically via xcex2-glucoside bonds between the 1-carbon of the first glucose group and the 3-carbon of the second glucose group; these bonds are consequently xcex2(1-3) bonds; they are linear polysaccharides which can carry xcex2 1-6 branches and which have up to 10,000 monosaccharide units per molecule.
The xcex2 1-3 glucans have various origins.
They can be extracted
from bacteria, in particular from Alcaligenes faecalis, the extract in question being named curdlan,
from fungi, in particular from Schizophyllum commune (the extract being schizophyllan), from Sclerotium glucanium (the extract being scleroglucan), other fungal extracts being pachyman, lichenan (extracted from Cetraria islandica), paramylon and lentinan (extracted from Lentinus edodes),
from yeasts, in particular from Saccharomyces cerevisae, the extracts thus obtained being yeast glucan and zymosan, and
from various plants, in particular from algae and from cereals, the corresponding extracts being denoted by xe2x80x9cseaweed xcex2-glucanxe2x80x9d and xe2x80x9ccereal xcex2-glucanxe2x80x9d, other extracts being callose (extracted, for example, from graminaceous pollen grains).
The xcex2 1-3 glucans can be hydrolysed enzymatically or with acid.
The xcex2 1-3 glucans can be enzymatically degraded by use of xcex2-glucanases extracted from yeasts of the Saccharomyces cerevisiae type or from mollusks.
Mention may be made, among these xcex2-glucanases, of EC 3.2.1.21 or EC 3.2.1.6.
The desired oligosaccharide fraction is isolated by ultrafiltration from the hydrolysate thus obtained.
It is pointed out that the oligo xcex2 1-3 glucans targeted in the context of the invention can also be prepared by chemical synthesis by employing the Koenig-Knorr reaction.
A first stage in this reaction consists of the acetylation of xcex2 D-glucose under warm conditions in sodium acetate (CH3COONa).
In a second stage, the xcex2 D-glucopentacetate thus obtained is subjected to the Koenig-Knorr reaction.